I think asking if it might be included for iPhone or iPad makes it okay....I know there are some apps that do this, but it would be a nice add for the iPhone.
Yes but I speculate Apple added it because they want to sell watches. Maybe it eventually comes to iPhone and iPad but if it does I bet it would be a year or two later (and will depend on whether it's a popular feature or not).
I have a watch but I would like this on the phone too. The current method of recording and sending voice messages in iMessage has always been a bit clunky to me. We rarely use it.
Hopefully Apple will make this available across all platforms, as it can be a useful feature.
In the meantime, for those seeking a free app with similar functionality, Voxer is decent, can be used as a Push To Talk Walkie Talkie and works on both iOS and Android.